This invention relates to well drilling and, in one aspect, to a portable rotary top drive assembly.
In conventional rotary drilling of wells, particularly oil and gas wells, the pipe string (to which is attached a drilling bit at the end) is rotated by means of a rotary table located on the platform floor. The pipe is connected to the rotary table by means of a special connector between the table and pipe--a kelly. Drilling is accomplished in increments of single pipe lengths. When drilling has advanced one pipe length, drilling is stopped, the pipe de-coupled from the kelly and another length (typically 31 feet or about 9.4 meters) re-coupled and drilling is resumed.
Systems for rotary drilling an entire stand (typically three pipe lengths, about 93 feet or 28.3 meters) from a drive unit suspended at the top of the pipe string (top drive units) have been devised in recent years. The concept of top drives on drilling rigs dates back to the 1920s, but practical systems date only from the 1980s. It is relatively easy to make a machine to rotate pipe from the top, and many attempts were made to apply the idea to oilfield drilling rigs. However, a major problem of such rotary drive systems is in providing means to prevent reactive rotation of the top drive. In conventional systems this is done by attachment of the rotary table to the rig floor. In top drives the drive must be prevented from rotating in reaction to the rotation of the pipe string through the entire travel of the drill string, virtually the length of the mast. This requires a special torque reaction system. Most early top drive designs were unsuccessful because they were unable to efficiently handle drilling tubulars (pipe or tubing) within conventional drilling rig architecture. That is, they could not easily accommodate connection and disconnection of pipe lengths, moving and handling of the pipe strings or provide a workable torque reaction mechanism. In the 1980s there was developed a top drive system complete with pipe handler (breakout wrench, lift tilt, overdrill provisions and inside well control) that overcame the pipe handling limitations. This system allowed the top drive to achieve its conceptual benefits of continuously drilling triple stands of pipe and circulating and reaming the hole while tripping in and out of the hole (removing drill pipe and reinserting pipe strings in the drilled hole). The system has proven technically and commercially successful, and as many as 300 units have been used. However, these units are heavy and require significant field modification of the derricks. Most are installed on offshore rigs because activity levels are high and offshore day rates (costs of the operations and crew) are sufficiently high to justify the high purchase and installation costs. Moreover, offshore derricks are typically large enough to accommodate the relatively large and heavy top drives and torque reaction assemblies. These systems have not received much use on land-based rigs for a number of reasons. Land rig day rates are usually too low to justify very high capital costs solely on the basis of time savings, and the systems are too large to fit into land rig masts without extensive modification. Land rig masts are not strong enough to withstand the drilling torque reaction induced by top drives without significant reinforcement. Moreover, rigs outfitted with these top drive systems cannot efficiently be moved and re-rigged as needed.
Thus, smaller, lighter and more compact systems were needed and have been developed. Since land rigs are typically portable, they are designed with only enough space inside the top of the mast structure to accommodate a traveling block and pipe handling facilities. Thus, installing a torque reaction system is a major problem. It is not feasible to have the mast structure itself absorb the torque reaction. To do so would require expensive stress analysis of the mast and modification and additions to the rig structure that would be not only costly but would reduce portability of the rig. The solution to this problem has been to provide a separate torque reaction guide, vertical structures designed to guide the top drive assembly over the top of the drill string and to absorb torque reaction. Since tying the guide to the mast has many of the disadvantages of using the mast itself to absorb the torque reaction, the guides must be supported in another way. U.S Pat. No. 4,865,135 describes a system having a pair of bushings encased in a torque case. One bushing is attached to the top drive frame and the other pinned to the other bushing and slidably attached to a vertical shaft.
An improved torque guide, in which the drive assembly is slidably attached to a vertical guide consisting essentially of a rectangular monorail, is described in U.S. Pat. No. 5,251,709 and its counterpart, U.K Patent No. 2,228,025, published Oct. 21, 1992. This system is mechanically simpler than other designs and is more easily installed in existing small land rigs. The guide is attached at the top to the derrick crown and at the bottom to the platform floor or to the strong structure of a lower section of the mast. This torque guide system is very successful in absorbing torque reaction and is finding significant use. However, it requires some field rigging and is normally customized to fit one rig only. It is, therefore, less than ideal for retrofit installation in rigs already in use. Transporting equipment and personnel to the field, less efficient field work relative to shop work and especially down time of the rig (with a 3 to 5 person drilling crew idled during rigging) are costly.
There are, however, many operating land rigs that can benefit from top drive systems and many that cannot be economically retrofitted with the previous systems. What is needed is a truly portable, self-contained system that can be installed in an existing small rig in minimum time and with minimum modification to the rig structure. Ideally, a portable system should be capable of installation and be operational in one day and have little, if any, custom fitting or rigging in the mast. It should be easily removable, easily transportable and capable of use in a variety of rig configurations. It is especially important that utility and service connections be self-contained in a portable top drive assembly unit. Often, connections for utilities (electric, air and hydraulic) and services (drilling mud and cooling air, etc.) are more time consuming than installation of the torque guide itself. Thus, a truly successful portable top drive system for use in land rigs needs to be lightweight, compact and adaptable to a variety of rig architectures. It should be self-contained in its structural elements and its utility and service connections.
This invention is such a portable top drive assembly system.